When submarine cable or pipe is paid out from a barge or other vessel, particularly into deep water, it is desirable to support the weight of cable or pipe in the water by means of buoys or floats so that the entire length of cable or pipe does not present a tension load to the cable or pipe itself at its uppermost point. This is particularly important where the cable or pipe has discontinuities such as splices or joints that may be damaged by an extreme tension. A particular problem exists for the laying of submarine power cable, which is considerably heavier than communication cable. Experience has shown that splices in such cable, even when the diameter of the splices is only slightly larger than that of the cable itself, may develop cracks in the lead sheath under the armor or distortion of the insulation when high tension is encountered while taking bends at the cable laying capstan or bow sheave, even though these splices can take the same bends without difficulty at low tension. The tensile load encountered in submarine power cable being laid to depths exceeding 500 meters, and in recovering such cables for repair and re-laying may well be 20-30 metric tons or greater if the cable is not properly buoyed. Under such tension, particularly in the case of paper-insulated power cables, sudden longitudinal movement of the conductor may occur with respect to the rest of the cable structure. Such movements are, of course, very damaging, particularly at splices and terminations. In relatively shallow water, it has been known, especially in the laying of pipe, to reduce the load by buoying the pipe with hollow spheres or drums spaced at distances sufficiently large to permit the pipe to sink, dragging the buoys down with it. At a sufficient depth, however, the strongest of hollow buoys that are light enough in intrinsic weight to have a lifting effect, would be collapsed by external water pressure. Even were this not the case it would be undesirable to support more flexible articles such as electric cables at great depths and when the articles are substantially horizontal, with the same initial buoyancy as was appropriate for the steep angle of descent a cable usually assumes immediately adjacent to the laying vessel. For highly buoyant floats that are attached at widely spaced points to a flexible article would raise the article at these points and cause it to festoon in the lengths between supports.
It has been suggested to buoy an elongated article from a series of boats or buoys floating at the surface and equipped with winches. The disadvantages of this system are that, in deep water, it requires an excessive quantity of supporting line, and that the small boats will transmit the vertical movement of waves and swells to the suspended article along with horizontal effects created by wind. For very long installations surface floats might constitute an intolerable interruption of normal surface navigation. It would appear desirable, therefore, to utilize a buoy that neither remains on the surface nor retains its full buoyancy while sinking to the sea bottom with the article being laid. It might be thought that a buoy in the form of a bell, open at the bottom, would supply the desired means since the equalization of pressure inside the bell would prevent its collapse and the compression of air in the bell would reduce its buoyancy as it descended. This simple expedient would be useless in deep water, however, since the loss of buoyancy, with the air volume halving in the first 10 meters of descent, is too rapid.
The vertical contour taken by a cable or pipe underwater as it is being laid is determined by the water depth, the speed of the laying vessel, and the rate of descent. This latter can be controlled by the selection of buoyancy devices and their spacing. The bottom contour of the ground underwater is known. But the vessel speed is subject to weather conditions. It would be of great advantage to make provision in the buoyancy devices themselves for differences in descent rates as required for different laying conditions. Under certain circumstances it will also be desirable to permit divers or submarines to control the release of buoying devices from an article being laid, at some precise moment.